1. Field of the Invention
The present invention relates to deslagging a steel making furnace using electrical current as a heat source to refine liquid steel and, more particularly, to such a furnace supported on a transfer car designed and constructed to tilt such a furnace toward a slag door end of the furnace for decanting slag and, if desired, tilt the furnace in an opposite direction for tapping a heat of steel during consecutive furnace cycles.
2. Description of the Prior Art
It is known in the art of steel making to use electric current as a heat source in a steel making furnace. Arc heating furnaces are used to heat a metal charge by either heat radiation from arcs passed between electrodes above the metal charge or by arcs passing from the electrodes to the metal charge where heat is generated by the electrical resistance of the metal charge. When the furnace has an electrically conductive furnace bottom, the bottom forms part of an electrical circuit powered by direct current. When the furnace has a non-conductive furnace bottom, the electrical circuit is powered by alternating current and the circuit is limited to the electrodes and metal charge. Induction furnaces are also used to heat a metal charge by using either inductors according to a transformer principle where the secondary winding is formed by a loop of liquid metal in a refractory channel or a coreless principle where induction coils surround the furnace wall and generate a magnetic field to impart energy to the metal charge in the furnace.
The present invention is applicable to such electric furnaces and in particular to an alternating current direct arc electric furnace equipped with three electrodes powered by three phase alternating current to establish arcs passed from an electrode to a metal charge, to another electrode and from electrode to electrode. The direct-arc electric-furnace as used in the steel industry is primarily a scrap-melting furnace, although molten blast-furnace iron and direct-reduction iron (DRI) are also used for charging the furnace. Combinations of scrap and minor quantities of blast furnace iron or direct reduction iron are common furnace charging compositions. A three-phase transformer, equipped for varying the secondary voltage, is used to supply electrical energy at suitable range of power levels and voltages. Cylindrical solid graphite electrodes are suspended by a mechanism from above the furnace downwardly through ports in a furnace roof to positions so that the electrodes conduct the electric current inside the furnace to maintain arcs for melting and refining a furnace charge. A sidewall supports the roof on a lower shell, which is provided with a refractory lining to contain the metal charge. In the electric furnaces known in the art, it is well known to pivotally support the lower furnace shell on a foundation and provide a furnace tilting drive to tilt the furnace in each of opposite directions for de-slagging and tapping. Other drive mechanisms are necessary to remove the roof from the upper shell to gain access to the furnace interior for the introduction of a metal charge.
It is known in the art to retain a quantity of the steel in the furnace after tapping which is commonly called a wet or hot heel practice. However, the structural integrity of the furnace mandates that the slag line be inspected periodically, typically every three to twelve heats with repairs performed based on the slag line condition. Generally, gunning will be performed several times a week. Periodically, every two-three weeks, the complete furnace bottom will be exchanged with a newly rebuilt bottom and worn bottom will have its sidewalls in the slag line area undergo a major repair or the complete refractory lining replaced.
The shortcomings of known electric furnaces are addressed in my patent application Ser. No. 09/737,440 filed Dec. 13, 2000 by the providing a transfer car to support a furnace shell with a sloping floor extending downwardly to a tap hole. A steel melt is refined in the furnace by maintaining a flat bath operation using electric current and the heat of a liquid hot heel consisting of at least 70% of tapped steel. Deslagging and tapping are performed while the lower furnace shell remains stationary at a furnace operating position on the transfer car. The lower furnace shell and if necessary the upper furnace shell or both the upper furnace shell and the furnace roof can be transported by a transfer car without the use of cars to a furnace component exchange position remote to the furnace operating position. This allows replacement of worn and damaged furnace components and quick reassembly of the furnace on the transfer car for movement back to a furnace operating position with a minimized down time.
The decanting of slag from the furnace on the transfer car is limited to the depth of slag at a slag discharge trough formed in the lower shell, vertically displaceable door is positioned to close the slag trough during refining of a steel melt. It is necessary to retain residual slag during tapping to control the metallurgy of the steel heat. The volume of the residual slag can be controlled by the height of the liquid steel in the furnace at the start of tapping. Such control is exercised by continuing the charging of scrap or other steel forming material until shortly e.g. three minutes, before tapping. However, such a measure for controlling the retention of residue slag introduces undesirable time constraints to the scheduling of tapping intervals.
Accordingly, it is an object of the present invention to tilt an electric steel making furnace while mounted on a transfer car stationed at a furnace operating site for decanting slag floating on the surface of liquid steel through a deslagging opening prior to tapping or at any other time during the furnace operation.
It is another object of the present invention to support a lower furnace shell of an integrated electric steel making furnace using a transfer car having a tilt drive for tilting the lower furnace shell while stationed at a furnace operating site in a manner to lower the refractory slag retention level at a slagging door end of the furnace for decanting slag.
It is another object of the present invention to downwardly tilt an electric steel making furnace carried on a furnace transfer car at the operating site for the furnace in an inclined fashion to lower the refractory slag retention level towards a slag door end of the furnace such that the electrode extending in the furnace, and other connections to the furnace permit the tilting of the furnace without disconnection or retraction of any connection particularly including the continuous supply of electrical power for operation of the electrodes while decanting slag and, if desired, while tapping of liquid steel heat.
It is another object of the present invention to tilt an electric steel making furnace mounted on a transfer car at a furnace operating site downwardly in an inclined fashion for deslagging and, if desired, for tapping by providing a flexible wheeled bogey assembly with a single pivot point controlled by a tilt arm at opposite sides at one end of the furnace and a pivot point fixed to the car frame at the opposite and non-operating end of the furnace.
According to the present invention there is provided an electric steel-making furnace including the combination of a furnace roof carried by an upper furnace shell on a lower furnace shell to substantially envelop an atmosphere above liquid steel and slag floating thereon in the lower furnace shell, the lower furnace shell having a refractory lining with a deslagging passageway in a sidewall thereof, a transfer car supporting the lower furnace shell for transport from a furnace operating site to a remote exchange site for exchanging one or more of the furnace roof, the upper furnace shell and the lower furnace shell, and a support including an actuator to tilt the lower furnace shell at the furnace operating site on the transfer car in a direction to increase the depth of slag at the deslagging passageway for decanting slag floating on liquid steel in the lower furnace shell.
Accordingly, the present invention also provides a method for producing steel in an electric furnace, the method of including the steps of providing an electric furnace including a transfer car supporting a lower furnace shell having a sidewall containing a deslagging passageway at an end of a floor wall opposite to a tap hole, refining consecutively steel heats in the electric furnace while residing on the transfer car at a furnace operating site, decanting slag floating on consecutively refined steel heats by using the transfer cat to tilt the lower furnace shell in a direction to increase the slag depth at the deslagging passageway, tapping liquid steel from consecutive steel heats after decanting slag there from by using the transfer car to tilt the lower furnace shell in a direction to increase the liquid steel depth at the tap hole, transporting the lower furnace shell on the transfer car from the furnace operating site to a remote lower furnace exchange site, using the transfer car to transport a replacement lower furnace shell to the furnace operating site, and thereafter repeating the steps of refining, decanting, and tapping.